WHO on Fukushima doses

24 May 2012

A preliminary report from the World Health Organisation (WHO) has estimated the radiation doses that residents of Japan have received in the year following the accident at Fukushima Daiichi. Dose rates in most Fukushima homes outside the 20 kilometre evacuation zone were comparable to reference levels for radon.

The nuclear accident at Fukushima Daiichi released significant amounts of radioactivity into the environment, with the majority of this dispersed by air along a track stretching about 50 kilometres to the northwest.

The WHO's study used radiological surveys and data available up to September 2011 and considered the location of people when the contamination occurred in March 2011. Japanese authorities acted quickly to evacuate people from within 20 kilometres of the plant and this uninhabited zone is the only part of Japan not covered by the study. Another 'deliberate evacuation area' was later declared to the northeast of the plant and it was conservatively assumed that residents stayed for four months before leaving.

The report noted that probably over 30% of the dose to Japanese citizens exposed to the contamination would have been received during the first year, with less than 70% to follow over a period of about 15 years. This is based on the experience of the Chernobyl accident of 1986, allowing for variation in the actual mix of isotopes released.

The report's headline conclusion is that most people in Fukushima prefecture would have received a radiation dose of between 1 and 10 mSv during the first year after the accident. This compares to levels of about 2.4 mSv they would have received from unavoidable natural sources, and the legal limit of 1 mSv from nuclear plants in normal operation. In two places the doses were higher - between 10 and 50 mSv.

In prefectures that border Fukushima (Miyagi, Yamagata, Nigata, Gunma, Tochigi and Ibaraki) people have likely received doses of between 0.1 and 10 mSv. Elsewhere in Japan the doses were between 0.1 and 1 mSv with the majority of this estimate coming from the assumption that they ate nothing but food sourced from Fukushima prefecture.

Around the world the dose from the accident's releases was put at below 0.01 mSv. This is "below (and often far below) the dose levels regarded by the international radiological community as very small."

Its conservative approach to food has resulted in "clearly overestimated" doses for Japan as a whole, said the WHO, which noted that direct measurements of residents had shown lower doses. But while it has confidence of its conservatism, the WHO said its estimates were "unlikely to be very significantly overestimated."

Thyroid doses

While contamination from caesium isotopes raises a person's general level of exposure, iodine-131 presents a special concern because it accumulates in the thyroid gland and this can lead to cancer in growing children. This is potentially fatal, although proper monitoring and treatment will cure over 95% of cases.

In the most affected areas of Fukushima prefecture thyroid doses from iodine-131 were estimated as between 10 and 100 mSv apart from one area which was lower, at 1 to 10 mSv, and one that was higher for infants at 100-200 mSv. In the rest of Fukushima, adults received 1 to 10 mSv to their thyroids while children and infants received 10 to 100 mSv, the study estimated.

For comparison, young people in the vicinity of the Chernobyl accident received on average doses of 300 to 1400 mSv to their thyroids.

Homes

The WHO put the radiation doses in Fukushima homes into context by comparing them to the levels of radiation permitted in homes from naturally occurring radon. It said, "In Fukushima prefecture and in neighbouring prefectures the estimated effective doses are below the internationally agreed reference level for the public exposure due to radon in dwellings (annual effective dose of about 10 mSv)..." This applies to all but two locations outside the evacuation zone where doses were higher at between 10 and 50 mSv. The WHO also noted that the ICRP recommends post-clean-up dose levels of 20 and 100 mSv as a reference for homes in emergency situations.

More to come...

All of the above comes from yesterday's release of a preliminary version of a WHO report to be completed in the latter part of next year. Also yesterday, the UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) announced it had a "good understanding of the nature and composition" of the releases from Fukushima Daiichi.

UNSCEAR has data from testing of 1000 children's thryoids, as well as 20,115 workers that have spent time at Fukushima Daiichi since the accident. This will be combined with measurements from the air, soil, water and food, to result in an UNSCEAR study that will "be able to assess doses to adults and children in different areas of Japan, considering important organs such as the thyroid." It will be presented to the UN towards the end of 2013.

In addition, a huge study is underway in Fukushima prefecture to log the movements of some 2 million people living there in March last year and evaluate their doses.

The trio of reports from the scientific community should inform governments of the effects of radiological contamination on a population and help inform future standards for evacuation and return.